Earliest Carboniferous tetrapod and arthropod faunasfrom Scotland populate Romer’s GapTimothy R. Smithsona, Stanley P. Wood1, John E. A. Marshallb, and Jennifer A. Clacka,2

aUniversity Museum of Zoology Cambridge, Cambridge CB2 3EJ, United Kingdom; and bOcean and Earth Science, University of Southampton, NationalOceanography Centre Southampton, Southampton, SO14 3ZH, United Kingdom

Edited by Neil H. Shubin, University of Chicago, Chicago, IL, and approved January 6, 2012 (received for review October 20, 2011)

Devonian tetrapods (limbed vertebrates), known from an increas-ingly large number of localities, have been shown to be mainlyaquatic with many primitive features. In contrast, the post-Devonian record is marked by an Early Mississippian temporalgap ranging from the earliest Carboniferous (Tournaisian andearly Viséan) to the mid-Viséan. By the mid-Viséan, tetrapods hadbecome effectively terrestrial as attested by the presence of stemamniotes, developed an essentially modern aspect, and given riseto the crown group. Up to now, only two localities have yieldedtetrapod specimens from the Tournaisian stage: one in Scotlandwith a single articulated skeleton and one in Nova Scotia withisolated bones, many of uncertain identity. We announce a seriesof discoveries of Tournaisian-age localities in Scotland that haveyielded a wealth of new tetrapod and arthropod fossils. Theseinclude both terrestrial and aquatic forms and new taxa. We con-clude that the gap in the fossil record has been an artifact ofcollection failure.

Ballagan Formation | end-Devonian mass extinction | terrestriality |rhizodonts | lungfish

Awide range of Late Devonian taxa from around the worlddocument the earliest phases of limbed vertebrate evolution

close to the fish-tetrapod transition. These animals do not closelyresemble modern forms but were fully or partially aquatic,retaining primitive features, such as fish-like bony fin webs on thetail, and where known, the limbs were paddle-like and poly-dactylous (1). Devonian limbed tetrapods were mainly around 1to 2 m in size with dorsoventrally compressed heads that werelarge relative to their snout-vent length (2). By the mid-Car-boniferous, terrestrial tetrapods had diversified into a wide rangeof morphologies and ecologies that are recognizably modern inaspect. By the mid-Viséan, limbs were penta- or tetradactylous,or secondarily reduced or lost, with small limbed forms about100 mm in length appearing (3). In the amniote stem lineage,heads had become smaller and deeper relative to snout-ventlength or skull width, a feature associated with the evolution ofcostal ventilation (2). Unfortunately, our understanding of thepatterns and processes behind their evolution and diversificationhas long been hampered by a gap in the continental fossil recordof many millions of years from the end of the Devonian (359 Ma)through to the mid-Viséan (∼365 Ma Early Carboniferous,Mississippian). The hiatus was recognized by A. S. Romer in1956 (4). At the time he was writing, it represented a period ofabout 30 million years covering almost the entire Tournaisianand Viséan stages. Although a few tetrapods from Asbian-age(late Viséan) localities were known (5), their aberrant aspect andaquatic habits did not cast direct light on the earliest phases oftetrapod terrestrialization. Over recent decades, the gap hasbeen narrowed to about 15 million years by the discovery ofmany new finds from the earlier half of the Viséan. Terrestrialassemblages of tetrapods from the mid-late Viséan (Asbian/Brigantian) have been discovered that, according to many anal-yses, include basal members of the crown group of tetrapods (6–10). Alternative phylogenies, despite differing in major respectsfrom the above, likewise place the origin of the crown group no

later than the mid-Viséan (11, 12). Among recent discoveriesfrom the mid-Viséan is the small amniote-like Casineria kiddi (3)documenting the earliest known pentadactyl forelimb. Thus, al-though the “gap” is closing, we have lacked information aboutthe crucial early part of the period during which terrestriality,defined simply as the ability to support the body and locomotecompletely out of water, may have been achieved.Among several explanations for the hiatus is one that took the

fossil record to reflect the actual pattern of evolution, marked bythe absence of animals from terrestrial ecosystems (13). Recentwork has also documented a dramatic changeover in globalvertebrate faunal assemblages, coinciding with the end-Devonianmass extinction (EDME) that wiped out many vertebrate groups(14). The EDME (Hangenburg crisis) was driven by a severeglacial episode during which regression and low-latitude conti-nental aridity were followed by a significant marine transgression(Hangenburg Black Shale) that caused ecosystem collapse(15). Among the vertebrate groups affected were many majoraquatic forms, such as the placoderms, acanthodians, and severalsarcopterygian groups, which, if not entirely exterminated,were seriously reduced in abundance (14). Subsequently, othergroups, such as actinopterygians, chondrichthyans, and ulti-mately tetrapods, increased in numbers and diversity to producea faunal balance more like that of today (14). How atmosphericor climatic conditions affected all these faunal changes remainsto be tested.The reestablishment of both aquatic and terrestrial ecosys-

tems, as well as their component faunas and floras, following theEDME is a phenomenon of great evolutionary significance. Bythe mid-Viséan, tetrapods that are generally considered to havebeen terrestrial had appeared (8–10) and the base of the crowngroup had been founded. Unfortunately, we are currently largelyignorant of the exact chronology and dynamics of these earlyphases of terrestrialization and diversification in tetrapod evo-lution. To discover significant numbers of fossil sites represent-ing this period would mark a major breakthrough in ourunderstanding of this crucial period in Earth history.

ResultsWe report the discovery of a suite of tetrapod fossils from fourlocalities in southern Scotland, dated as Tournaisian (16–19), ofwhich three have also yielded terrestrial arthropods. These lo-calities are the coastal section at Burnmouth north of Berwick onTweed, the banks and bed of the Whiteadder River nearChirnside, the banks of the Tweed River near Coldstream (all

Author contributions: T.R.S. and S.P.W. designed research; T.R.S., S.P.W., J.E.A.M., and J.A.C.performed research; T.R.S., S.P.W., J.E.A.M., and J.A.C. contributed new reagents/analytictools; and T.R.S., S.P.W., J.E.A.M., and J.A.C. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

Data deposition: Some fossil material has been placed in the University Museum ofZoology, Cambridge, and some will be placed in the National Museum of Scotland.1Present address: Rockville, 44, Hillside Terrace, Selkirk TD7 4ND, United Kingdom.2To whom correspondence should be addressed. E-mail: j.a.clack@zoo.cam.ac.uk.

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Tweed basin, Borders Region), and the sedimentary strata nearTantallon Castle on the south shore of the Firth of Forth(Midland Valley basin, East Lothian Region) (Fig. 1). In addi-tion, two previously known Tournaisian localities have yieldedtetrapod fossils: Dumbarton in western Scotland and Blue Beachnear Horton Bluff in Nova Scotia. These augment the scope ofour discoveries and help to give a fuller picture of events. Ourdiscoveries strongly suggest that the apparent gap in the fossilrecord does not reflect an accurate pattern of evolutionaryevents but has been the result of the lack of, or at least the lack ofsuccessful, collecting effort from appropriate sedimentary rocksand localities (16, 17).All but the Nova Scotia site derive from the Tournaisian ar-

gillaceous dolostone-bearing mudstone succession of Scotland.Formerly known as the Cementstone Group (the lower part ofthe Calciferous Sandstone Measures in Scotland), these rockshave now been assigned to the Ballagan Formation [InverclydeGroup (19)]. Most belong to the claviger-macra (CM) paly-nozone, dated as 348 to 347.6 Ma (20). The Blue Beach depositsare dated as pretiosus-clavata (PC) palynozone, late Tournaisian(21), approximately contemporary with or slightly earlier thanthe Scottish localities.

Burnmouth. The sedimentary rocks consist of a succession ofnear-vertical strata passing up the sequence from west to east.The oldest bed so far dated is verrucosus-incohatus (VI) paly-nozone, and the sequence records a conformable (althoughcondensed) series of time slices through the Tournaisian, al-though the Devonian-Carboniferous boundary and the base ofthe Tournaisian may not be present. An Old Red Sandstonefacies (possibly Kinnesswood Formation) marks the base of thesuccession, but its relation to the Tournaisian strata is uncertain.These strata are overlain by the Viséan Fell Sandstone Forma-tion. The Ballagan Formation as exposed at Burnmouth domi-nantly comprises mudstone, with interbedded sandstone and thinbeds of “cementstone,” and represents cyclical deposition. Plantmaterial from some beds has been described previously (22), andthe presence of a few “fish” fragments has been noted (19, 23).

The sequence has now yielded isolated and partially associatedtetrapod, lungfish, and arthropod material, including, unusuallyfor Early Carboniferous deposits, small individuals. Tetrapodremains have so far been recorded in five different horizons incirca 40 m of strata exposed in the upper third of the sequence.The most significant of these is a small (10 mm across the met-apodial series) pentadactylous autopod (identity as manus or pescannot yet be determined). It derives from a horizon a fewmeters above one that has been dated as CM palynozone. Itsmorphology strongly suggests that its owner was a terrestrialtetrapod [specimen University Museum of Zoology Cambridge(UMZC) 2011.8] (Fig. 2). The proportions of the metapodialsand preserved phalanges, being elongate and gracile, are mostsimilar to those of the Viséan forms Silvanerpeton, Eldeceeon,and Balanerpeton, as well as the Late Carboniferous anthraco-saur Gephyrostegus, all of which are usually considered to havebeen terrestrial (8, 24). They are quite unlike those of otherEarly Carboniferous forms, such as Pederpes (25, 26), Crassigyr-inus (27), Greererpeton (28), or Proteroygyrinus (29).Two or three meters above this, a partial Crassigyrinus-like jaw

(specimen UMZC 2011.9.1) (Fig. 2), a large rhizodont shouldergirdle (specimen UMZC 2011.9.3), numerous large lungfishelements (e.g., specimens UMZC 2011.9.6–7), and Gyracanthusspines have been found, together with a wealth of fossil charcoal.Crassigyrinus is a large tetrapod previously known only from thelate Viséan and early Namurian of Scotland (30, 31). The newlydiscovered jaw ramus is almost exactly the same size as theknown specimens, has almost identical external ornamentation,

Fig. 1. Map of southern Scotland and northern England to show the loca-tions of Tournaisian sites in Scotland. Large gray circles represent sites, andsmall gray circles represent other towns.

Fig. 2. Burnmouth specimens. (A and B) Pentadactylous autopod (specimenUMZC 2011.8). A and B are to the same scale. (A) Photograph. (B) Interpretivedrawing. (C) Photograph of a Crassigyrinus-like partial lower jaw (specimenUMZC 2011.9.1) in an external view. Anterior end and tooth-bearing bonesare missing. (D) Interpretive drawing of the specimen. (E) Reconstruction ofthe skull of Crassigyrinus scoticus from the Viséan of Scotland (based on ref.31) to the same scale as the lower jaw in C.

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and differs from the known specimens in only minor details ofthe internal structure. This gives Crassigyrinus a temporal dis-tribution comparable to that of the whatcheeriid family, whichoccurs at Blue Beach (32). This Burnmouth horizon confirms thepresence, by this early date, of large vertebrates whose affinitiesare with later Carboniferous rather than Devonian forms.

Willie’s Hole Near Chirnside. Gently dipping Ballagan Formationstrata, consisting of sandstones, mudstones, shales, and thincementstones, crop out in the bed of the River Whiteadder (19).From it, three separate conformable horizons have yielded a wealthof tetrapods, rhizodonts, lungfish, actinopterygians, Gyracanthusspines, myriapods, crustaceans, scorpions, and eurypterids. Thebase of the tetrapod-bearing sequence is the Willie’s Hole ShrimpBed (33). This gray mudstone contains predominantly plantremains (bed 3). A second gray mudstone unit separates it from anoverlying second tetrapod-bearing horizon (bed 2). This dark silt-stone contains tetrapods, lungfish, rhizodonts, and arthropods, therichest of the three horizons. Above that is a third horizon yieldingmainly actinopterygians (bed 1, Fig. 3 A and B). The site is about 2miles from the contemporary locality of Foulden, which does notpreserve tetrapods but includes many fish taxa common betweenthe two sites (34).Dated as the lower part of the CM zone, the Willie’s Hole

succession represents a whole ecosystem changing through time.The tetrapod collection includes about 100 samples of large andsmall semiarticulated tetrapod skeletons and isolated bones.One of the small individuals (specimen UMZC.2011.7.2, new

taxon A) is one of the few tetrapod specimens to have beenrecovered from the horizon of bed 1. A small individual, it

preserves a lower jaw, representing a skull about 75 mm long,with enough of the cheek preserved to allow a provisional skullreconstruction (Fig. 4). There are also two sclerotic plates andnumerous elongate gastral scales. The reconstructed skull hasa deep quadratojugal, a large squamosal, and a narrow suborbitalprocess to the jugal underlying a large orbit, consonant with the4-mm-long sclerotic plates. The snout is short, and the teeth areconical but with “labyrinthodont” enamel. Its proportions mostclosely resemble those of the Viséan Silvanerpeton (35) or theLate Carboniferous Gephyrostegus (24). The specimen preservesa disrupted but probably pentadactyl manus, of which themetacarpal and phalangeal proportions are quite different fromthose of specimen UMZC.2011.8, in being short and squat (Fig.4). These proportions also rule out its identity as either of thelatter named genera. Its distinct dermal sculpturing and well-ossified phalanges suggests that it was not a juvenile of one of thelarger forms.Bed 2 has yielded by far the majority of tetrapod remains, in-

cluding several semiarticulated skeletons and numerous isolatedbones.One of the largest specimens [specimen from the collectionsmade by S.P.W. (SPW) 4165, new taxon B] from this bed preservesa disrupted postcranial skeleton with conspicuous curved ribs, partof a forelimb, and both hind limbs, with some pedal elements anda few associated skull bones (Fig. 3 D and E). Although the hindlimb elements show some similarities to those of Pederpes, the ribsare not flanged as they are in that taxon, and they are longer andmore robust in specimen SPW 4165 (26). The hind limb elementsare larger, relative to the ribs, than they are inCrassigyrinus (27). Asthe only two comparable taxa from the Early Carboniferous, thesedifferences rule out specimen SPW 4165 as a member of either ofthese genera, but only further study will elucidate their relation-ships. A complete description of this specimen should help resolvesome of the conflicting positions for Pederpes and Crassigyrinus atthe base of the stem tetrapod tree (6, 7, 35).An almost complete skull roof is represented by specimen

SPW 4034, and it probably represents a further new taxon (Fig.3C). It has a rounded but short snout with large orbits and or-namentation reminiscent of that of a temnospondyl, but onlyfurther study could confirm or refute such an assignment. Ifcorroborated, it would represent the earliest member of thegroup by about 15 million years.The myriapods (specimen SPW 762 and specimens UMZC

2011.7.2–3, Fig. 5) are among the earliest known from the Car-boniferous and probably include two new taxa. The presence ofmyriapods demonstrates the potential of the site to preserve ter-restrial elements of the fauna. Scorpions (specimen SPW G765,Fig. 5), eurypterids, and myriapods have been recovered from bed2. Bed 3, in addition to plants, yielded tetrapods, a rhizodont,actinopterygians, crustaceans, and two small gastropods.

Coldstream. Fossils include tetrapod skull bones and vertebrae, aswell as lungfish and rhizodont elements, Gyracanthus spines, andscorpion fragments. The nearby locality of Lennel Braes hasyielded the myriapod Anthracodesmus, previously dated asViséan (36) and considered as such by Ward et al. (13). The mostrecent survey by the British Geological Survey has redated thesequence as Tournaisian (37).

Tantallon Castle, East Lothian. Sedimentary rocks close to the fossilplant localities of Castleton Bay and Oxroad Bay (38) haveyielded isolated vertebrate bones and teeth, including tetrapodelements, such as a partial large lower jaw (specimen NationalMuseum of Scotland G 1977.43.3) (39).

Dumbarton, Western Scotland. Pederpes finneyae, represented by analmost complete articulated skeleton, is an isolated occurrence inthe Ballagan Formation of the area (25, 26). Virtually no othertetrapod or even vertebrate elements have been found there. Its

Fig. 3. Willie’s Hole specimens. (A) Actinopterygian Phanerosteon ovensi(UMZC 2011.7.11). (B) Actinopterygian Aetheretmon valentiacum (UMZC2011.7.12). (C–E) Willie’s Hole Level 2 specimens. (C) Skull in natural mouldshowing orbit and outline of skull (SPW 4034). (D) New tetrapod taxon B“Ribbo” (SPW 4165). (E) Inset showing femur, tibia, and fibula.

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phylogenetic position is basal to almost all post-Carboniferoustetrapods, with the possible exception ofCrassigyrinus (6, 7, 31, 35),and it appears to have had hind limbs adapted for walking on land(26). In part, its importance lies in providing identification of iso-lated elements from other localities, including Blue Beach (32).

Blue Beach, Nova Scotia. Known for many years, this locality hasyielded isolated elements, some resembling Pederpes and othersthat are more similar to Devonian forms (32, 40). They mayindicate that some known Devonian tetrapod taxa survived theEDME. Equally significant, many horizons yield subaeriallyproduced pentadactyl and tetradactyl tetrapod trackways repre-senting at least six different forms of both large and small ani-mals. Their clarity and depth indicate that the animals couldwalk unsupported by water. These attest to the great diversityreached by terrestrially capable tetrapods as early as the PC zone(41, 42). There are also large rhizodont and lungfish elementscomparable to those in Scotland, as well as actinopterygians.Identification of the some of the isolated bones can be aided bydescription of the recently discovered Scottish material.

DiscussionThe hypotheses put forward by Ward et al. (13) should, in future,be testable against our new records. Those authors associated theabsence of fossil tetrapods from this period with a correspondingabsence of terrestrial arthropods. By estimating the diversity andorigination rates of tetrapods (“stegocephalians” in the authors’terms) and arthropod groups during the Early Carboniferous, theysuggested that the lack of fossils was related to low atmosphericoxygen levels pertaining at that time, as modeled by Berner (43).These low levels, it was postulated, inhibited the animals fromleaving the water because of poorly developed respiratory struc-tures, and also restricted their diversity.

Our discoveries clearly demonstrate that multiple localitiesyielding terrestrial arthropods and probably terrestrial tetrapodfossils from the Tournaisian do exist, thus providing evidencethat the gap was indeed the result of collection failure. Thecontention in the study by Ward et al. (13) that tetrapods mighthave been confined to the water during this time does not explainthe existence of the hiatus in fossil discoveries. Most fossil tet-rapods are found in deposits laid in aquatic environments, fromwhich they are more, rather than less, likely to be recovered.Furthermore, our recent finds suggest that there may be no needto postulate low atmospheric oxygen levels at this time to explainthe absence of fossils. The discovery of charcoal-bearing beds inthe Burnmouth sequence challenges some of the implications ofthe study by Berner (43) with respect to oxygen levels (44).Our new records, combined with those from trackways, sug-

gest that tetrapods appear to have recovered relatively rapidlyfrom the EDME by the mid-Tournaisian. Fish groups hadevolved, or reevolved, into new large forms (e.g., rhizodonts,lungfishes). By the mid-Viséan, not only had tetrapods appearedthat are usually considered terrestrial (8–10, 35) and the base ofthe crown group been established (6–12), but highly specializedsecondarily aquatic forms had also evolved (45).By examining the earlier parts of the Burnmouth sequence, we

can test the hypothesis of the “Lilliput effect” (46) on vertebrateevolution; that is, following the EDME, vertebrates underwentan evolutionary stage constrained to small size by adverse con-ditions, such as aridity or other climatic conditions. We canprovide a maximum time (less than 10 million years) by whichthey had overcome such a constraint.Rather than beginning immediately following “Romer’s Gap,”

we can now test the hypothesis that diversification and terres-trialization of tetrapods had been taking place during the 15 ormore million years that it represents. Our discoveries and other

Fig. 4. Willie’s Hole, new tetrapod taxonA: UMZC 2011.7.2. A photograph of partof the specimen, with interpretive dia-grams from part and counterpart, isshown. (A) Reconstruction of skull. (B) Di-agram of the cheek and lower jaw used tocreate the reconstruction, in position onthe specimen. (C) Two sclerotic plates. (D)Digit elements as distributed on the spec-imen. (E) Radius and ulna. In A and B, thegray shading indicates the natural mold.

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recent new records from elsewhere certainly suggest that manytetrapod lineages have their origins much earlier than previouslyappreciated, and their earliest appearances may well be extendedback in time as the result of further research. Fig. 6 showsa family tree against a Carboniferous time scale taking the re-cently discovered data into account.These new finds will ultimately throw light on the ecological and

environmental circumstances under which the establishment ofmodern terrestrial faunas took place. The discoveries will allow usto test the hypothesis put forward by Ward et al. (13), followingcareful phylogenetic as well as diversity and disparity analyses en-visaged for the future. These finds will allow us to put forward

refined hypotheses, testable by further finds and analyses. Thewealth ofmaterial from several different sites and environments willprovide the opportunity to investigate the causes and consequencesof the EDME. Our initial results suggest that reestablishment of atleast some components of the tetrapod fauna was achieved within10 million years.We have established that pentadactyly arose about20 million years earlier than previously documented (3). Studiesmay now examine the interlinkage of environmental and atmo-spheric changes to faunal turnover, the timing of ecosystem re-covery, the sequence of acquisition of terrestrial characters bytetrapods, resolution of the problems of relationships among earlytetrapods (and thus the recalibration of the phylogenetic tree), andthe time of appearance of crown group tetrapods, based on thepresence, rather than the absence, of fossil data.

Materials and MethodsT.R.S. prospected, collected, and prepared material, using mechanical andacid digestion techniques, from Burnmouth and Coldstream over a period of23 y, starting in 1988. S.P.W. prospected, collected, and prepared material,

Fig. 5. Willie’s Hole arthropods. (A–C) Myriapods. (A) Specimen SPW 762. (Band C) Specimens UMZC 2011.3–4 in part and counterpart. (D) Scorpionspecimen SPW 765.

Fig. 6. Carboniferous time scale with superimposed family tree of tetra-pods, including recent data on occurrences and allowing for uncertainty inparts of the tree. Arrows at the top of lines indicate that groups persistedinto the Permian. The Devonian tetrapod tree is based on the study byCallier et al. (47). The Carboniferous tetrapod tree (6, 7, 35) is shown with thenode for the origin of the crown group on those phylogenies marked by A.Alternative phylogenies (11, 12) place the node for the origin of the crowngroup at B. All phylogenies imply an origin for the crown group no laterthan the early to mid-Viséan. Casineria is placed incertae sedis among thestem amniotes, and the Burnmouth foot is placed outside the tree. What-cheeriids may extend into the Late Devonian (48), “microsaurs” into theBrigantian (49), and Crassigyrinus into the Tournaisian (this paper). Dating isaccording to the study by Gradstein et al. (20). Ad, adelogyrinids; Am,amiotes; An, anthracosaurs; Ai, aïstopods; Ba, baphetoids; BB, Blue Beach;Bm, Burnmouth; BmF, Burnmouth foot; Ca, Casineria; Co, colosteids; Cr,Crassigyrinus; Di, diadectids; EK, East Kirkton; Ge, gephyrostegids; Mi,microsaurs; Ne, nectrideans; Se, seymouriamorphs; Te, temnospondyls; Wh,whatcheeriids; WH/Db, Willie’s Hole/Dumbarton.

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using mechanical, airbrasive, and acid digestion techniques, from Willie’sHole, Burnmouth, and Tantallon Castle. Willie’s Hole material was recoveredby him from the bed of the River Whiteadder between 2008 and 2009. J.A.C.and R. N. G. Clack collected and prepared material, using mechanical tech-niques, from Burnmouth in 2010 and 2011. J.E.A.M. provided paleontolog-ical analyses and dating of the new sites. Rough-crushed dark mudstonesamples were treated with 30% HCl, followed by decant washing with waterto neutral and then 60% HF, a further decant washing to neutral, and thensieving at 15 μm. Neoformed fluorides were removed by a single shorttreatment with hot HCl, followed by a further sieving. The organic residuewas then permanently mounted using Elvacite 2044 (Lucite InternationalLtd.). Photographs were taken by S.P.W., J.A.C., J. Gundry, and R. Stebbings(UMZC), and figures were prepared by J.A.C. Those specimens not already in

the possession of the UMZC are being purchased and will be housed by theNational Museum of Scotland.

ACKNOWLEDGMENTS. We thank C. Macfadyen and Karen Rentoul (ScottishNatural Heritage) for permission to collect at a Site of Special ScientificInterest and advice about Burnmouth collections, C. Maclean for permissionto collect on his land, and D. Allan and P. Allan for permission to collect fromthe cliffs at Burnmouth. We acknowledge the support for this project ofOliver Keiran and the Burnmouth community. Sonja Wood and Chris Mansky(Blue Beach Museum) kindly allowed J.A.C. to examine Blue Beach speci-mens, and Deborah Skilliter (Nova Scotia Museums) gave permission to studythem. We thank David Millward (British Geological Survey), Nicholas Fraser(National Museum of Scotland), and two anonymous referees for helpfulcomments on an earlier draft of the manuscript.

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